In radio telecommunication systems of the time division multiple access type (TDMA) the communication on the radio path is carried out time-divisionally in successive TDMA frames each of which consists of several time slots. A short information packet is transmitted in each time slot as a radio frequency burst having a finite duration and consisting of a group of modulated bits. Time slots are mainly used for carrying control channels and traffic channels. Speech and data are transmitted on traffic channels. Signalling between a base station and mobile subscriber stations is carried out on the control channels. One example of a TDMA radio system is a pan-European mobile communication system GSM (Global System for Mobile Communications).
In traditional TDMA systems each mobile station is allocated a single traffic channel time slot for data or speech transmission. Thus in the GSM system, for example, a common carrier wave can carry up to eight parallel connections to different mobile stations. The maximum data transfer rate on one traffic channel is limited to a relatively low value according to the available bandwidth and channel coding and error correction used in the transmission, e.g. 9.6 kbit/s or 12 kbit/s in the GSM system. In the GSM system a half-rate (max. 4.8 kbit/s) traffic channel can also be selected for low speech coding rates. A half-rate traffic channel is established when a mobile station operates in a specific time slot only in every second frame, that is, at half rate. Another mobile station operates in every second frame in the same time slot. Thus in terms of the number of subscribers, the capacity of the system can be doubled, that is, as many as 16 mobile stations can operate on the same carrier at the same time.
In recent years the need for high-speed data services has grown significantly. For example, transmission rates s of 64 kbit/s or higher would be needed for utilizing the circuit switched digital data services of the ISDN (Integrated Services Digital Network). Higher transmission rates, such as 14.4 kbit/s, are needed for data services of the public switched telephone network PSTN, such as modem and class G3 telefaxes. Mobile video service is one area of growth in mobile data transmission that requires higher transmission rates than 9.6 kbit/s. Security surveillance by cameras and video databases are examples of these services. The minimum data rate in video transmission may be 16 or 32 kbit/s, for example.
The transmission rates of present mobile communication networks are not, however, sufficient for satisfying these new requirements.
One solution, disclosed in Applicant's co-pending Finnish Patent Application 942,190, which was unpublished at the filing date of the present application, is to allocate to one high-speed data transmission two or more parallel traffic channels on the radio path. A high-speed data signal is divided into these parallel traffic channels at the transmission end for the transmission over the radio path and it is reassembled at the reception end. Thus data transmission services are provided in which, depending on the number of the allocated traffic channels, the transmission rate may be up to eight times higher in comparison with the standard transmission rate. For example, in the GSM system two parallel traffic channels provide a transmission rate of 2.times.9.6 kbit/s which is sufficient for a modem or a telefax of 14.4 kbit/s. Six parallel traffic channels will enable a transmission rate of 64 kbit/s.
In using parallel traffic channels the problem is how to divide the data flow between parallel transparent traffic channels and how to synchronize the reassembling of data received from the parallel traffic channels.